The overall objective of this protocol is to determine whether exposure to a 3-hr pulse of bright light or physical exercise can phase-shift circadian rhythms. We will use two novel procedures to delineate the phase-shifting effects of these 2 stimuli presented on the 1st day following entrainment in normal subjects. To estimate circadian phase positions, we will monitor simultaneously the rhythms of plasma cortisol, TSH, melatonin, glucose regulation, body temperature, heart rate and blood pressure.